1. Field of the Invention
The present invention relates to improvements in a structure for supporting an engine upon the frame in a power unit swing type motorcycle.
2. Description of Relevant Art
A motorcycle of the power unit swing type is well known in the relevant art. In such type of motorcycle, the power system of an engine, a crankcase and a transmission case is integrated into a unit. The front portion of the unit is pivotally connected to a frame, and the rear portion of the unit is employed to support a rear wheel and is connected to the frame through a shock absorber. The unit is subjected to rocking motions about the pivot at the frame, together with the rear wheel.
There is one type of such a power unit wherein the transmission case is arranged in the longitudinal direction of the vehicle, while the engine is arranged in an upright position, i.e., substantially in a vertical orientation. In such a power unit, the main axis of inertia of the engine is substantially vertical so that the reciprocal movements of the engine piston as well as the main vibrating forces caused by rotational inertia of the crankshaft are transmitted as vertical vibrations to the frame, thereby deteriorating the riding quality of the motorcycle. If the motorcycle is parked with its stand in a lowered position while the engine is operating, the frame is vibrated in a vertical direction by the vertical vibrations of the engine. As a result, it is possible that the frame will be sufficiently vibrated up and down as to undesirably move the motorcycle.
In consideration of such problem, there has been provided an engine supporting system of the power unit swing type having the engine thereof arranged in the longitudinal direction so that the main axis of inertia thereof may be substantially longitudinal, i.e., horizontal. The power unit of this type is also supported upon the frame by pivotally connecting the engine to the frame at only one point by means of a pin. As a result, the power unit is mounted so as to theoretically accommodate the pivotal up and down motions about the pivot for the shock absorbing actions of the rear wheel, however, it is substantially rigid with respect to pivoting in the longitudinal direction. Therefore, in the event that the rear wheel rides over a projection on a road, it moves upwardly but not back and forth, and the resultant shocks will be transmitted to the frame. Thus, it is difficult in accordance with such known structure to obtain a satisfactory compliance of the rear wheel in the longitudinal direction, which compliance is expected as an important function of a suspension system of a motorcycle.
It is known to mount rubber bushes on the pivots of the engine to the frame, however, such rubber bushes are required to prevent vibrations as well as to bear the load of the power unit. Accordingly, the characteristics of the rubber bushes are at best only moderate with respect to both the vibration-preventing performance and the load bearing performance thereof. It is thus quite difficult to obtain sufficient vibration preventing characteristics and to satisfy the load bearing and shock absorbing characteristics of the power unit. It is also difficult to provide a power unit which provides sufficient compliance characteristics to the rear wheel in the longitudinal direction.
The present invention provides an effective and simplified solution to the aforementioned engine supporting problems which are encountered in a motorcycle of the power unit swing type.